You just go and be a contractor. There are tons of contracting positions open in London that will pay £500 - £600 per day. It's easy to make over £100k (pounds) if you're a good contractor.
This is just standard contracting where you apply for jobs that only last a certain length of time. This isn't uncommon in the UK IT industry.
I notice some Americans will call freelancing contracting even though they mean two very different things over here.
As a contractor, you generally have several years of experience in a technology (let's say Oracle ATG ecommerce platform). Companies that want to implement ATG may not want to hire a full time and permanent group of Software Engineers as building software is not there business, instead, they will hire a contracting firm, someone like IBM, Oracle or Accenture. These firms send people to the client to work in the new implementation on contracts that could last from 1 month to 12 months (for example).
Sometimes companies and consultancies will hire independent consultants (like the parent poster) who will join a company for a just a few months / until the project is done then they will go and join a new company so on and so forth.
Contractors generally charge per day (a good friend of mine charges £500 per day) and they have no employee benefits.
It's a tough market but if you have the right skills (tech and otherwise) you can earn yourself some very good money.
Some Americans think that there freelancing efforts are the same as contracting. This may be the case in some instances but a lot of the time I see people claiming that you don't need much experience go contracting and it's easy work to get if you try hard etc etc. These are usually Americans that are getting confused between the professional service offering that is contracting and the low value informal work they do which I would call freelancing.
EDIT: the parent just told us he was talking about a cleaning business. What I have described is IT contracting.
http://www.cwjobs.co.uk/JobSearch/Results.aspx?Keywords=core...